In the refueling of aircraft fuel tanks, for example, it is one common practice to couple the ends of a fuel supply line to an aircraft fuel tank and to a hydrant valve. In such system, when the fuel line coupler is coupled to the hydrant valve, the opening of the coupler valve simultaneously opens the hydrant valve for flow of fuel under pressure from an underground storage tank through the fuel supply line into the aircraft fuel tank. In known systems of this character as disclosed, for example, in the U.S. Pat. Nos. 3,221,764; 3,286,725; 3,298,390; and 3,351,094, the pressure control valves or regulators are embodied in the hydrant valve unit and hence must be duplicated in each installation at substantial cost.